Ion Mobility Spectrometer coupled with electrospray ion source and interfaced to a Mass Spectrometer is a powerful tool for the study of molecular conformations, separation of mass isomers, and analysis of complex mixtures and suppression of chemical background. One of the reasons that limit the capabilities of the technique is its relatively low resolving power. The gas-phase diffusion results in the peak broadening and limits the resolution at 50-100 level. We propose the alternative principle of gas-phase ion separation, based on ion focusing under the influence of uniform electrostatic field and decelerating gas flow. Both analytical calculations and numerical simulation show that a diffusion-limited resolution of several hundred can be achieved. We propose to build an ion mobility analyzer of the new type that is called Focusing Differential Mobility Analyzer (FDMA). The proposed prototype FDMA can be easily interfaced with commercial mass spectrometers and offers the theoretical resolution of several hundred and ion transmission close to 100%. The purpose of this proposal is the demonstration of the feasibility of atmospheric-pressure ion separation based on the new principle of ion focusing under the influence of uniform electrical field and decelerating gas flow. A prototype FDMA apparatus will be designed and built. The electrospray ion source will be coupled with FDMA. The devices will be interfaced with commercial LCQ ion trap of Thermo Finnigan, Inc., installed in our laboratory. The success of the research will provide a scientific community with a new class of ion mobility analyzer that combines better resolution and sensitivity with simplicity of design and interface to mass spectrometers of different types. [unreadable] [unreadable] [unreadable]